1. The Field of the Invention
The present invention is related to optical devices for measuring light. In particular, the present invention is related to a compact spectrometer device for use in color discrimination or evaluation.
2. The Relevant Technology
Optical devices known generally as spectrometers have been developed for measuring and analyzing the spectral or color content of electromagnetic radiation in the frequency range or spectrum of optical wavelengths. These include from ultraviolet, through visible, to near-infrared wavelengths, which include the portion of the electromagnetic spectrum producing photoelectric effects, referred to herein as "light." Various kinds of opto-electronic devices are used for both imaging applications, as by inspecting the spectral reflectance characteristics of a two-dimensional object, and for non-imaging applications.
Spectrometric measurements of light are performed in basically two ways, including dispersion-based techniques and filter-based techniques. In the dispersion-based approach, a radiation dispersion device such as a prism or diffraction grating is used to separate the incident polychromatic light into its spectral contents. The spectrally separated light is then projected onto a photodetector to measure the relative intensity in each spectral range. While dispersion-based devices can be effectively used in some applications, they have the disadvantage of being easily knocked out of alignment during use, and thus not suitable for more rigorous applications in the field.
In the filter-based approach to spectral measurement, various types of optical filters are used in conjunction with photodetectors to measure and analyze light. For example, in one approach, a single band-pass filter is placed over a detector to measure a single spectral band of the incident light. In another variation of the filter-based technique, a filter wheel on which several filters are mounted is used in conjunction with a single photodetector or several photodetectors. Alternatively, the discrete filters in the filter wheel can be replaced with a continuous circular variable filter (CVF) which is placed over a detector. Further, the CVF may be placed over several detectors to provide simultaneous spectra in a limited number of bands. These filter-based techniques are limited for practical reasons to use in low resolution spectral measurements of a few bands of light and to non-contiguous bands only.
Other spectrometer devices have been developed that utilize linear variable filters in an attempt to enhance light measuring capabilities. For example, U.S. Pat. No. 5,166,755 to Gat discloses a spectrometer apparatus including a spectrum resolving sensor containing an opto-electronic monolithic array of photosensitive elements which form a photodetector, and a continuous variable optical filter such as a linear variable filter (LVF) that is placed in an overlaying relationship with the photodetector. The LVF and photodetector are mounted in a single housing which serves to support at least the filter and the array in a unitary sensor assembly. The LVF is formed by depositing optical coatings directly onto the photodetector array, or a preformed LVF may be positioned in contact with or slightly above the array.
In U.S. Pat. No. 5,218,473 to Seddon et al., a leakage-corrected linear variable filter is disclosed. This patent describes a conventional linear variable filter system as including an LVF positioned in a spaced apart relationship with a linear detector such as a charge coupled device array. The LVF is paired with a linear detector having comparable dimensions in order to form a detector capable of receiving and discriminating a number of wavelengths of radiation simultaneously.
The use of linear variable filters in spectrometer devices has been limited because of fundamental packaging problems. Depositing an optical coating on the surface of a detector to form an LVF is problematic because of the delicate surface and wiring of the detector array. The placement of a preformed LVF on the surface of the detector array requires the removal of a cover glass that protects the delicate surface of the detector array. Such placement of an LVF during manufacture can damage the surface of the detector array.
Further, the LVF is prone to have diffuse leaks that downgrade its spectral performance and which are unavoidable when the LVF is placed in contact with the detector array surface. The LVF filter works properly only within a limited cone angle of light (numeric aperture). Light outside this limited angle may contain diffuse leaks. The detector array is capable of receiving light within the full hemisphere and will detect the diffuse leaks when placed in contact with or very close to the LVF. In addition, if the LVF is spaced apart from the surface of the detector array, then the LVF will not perform properly since light emitted from one position of the LVF may reach more than one element of the detector array, thereby limiting the spectra resolving power of the LVF.
Accordingly, there is a need for an optical/detector device that overcomes or avoids the above problems and limitations.